Heart disease and cancer, the top two killers of Americans, account for just under half of all U.S. deaths.

The new numbers come from the CDC's preliminary analysis of 2008 death certificates, the most recent data available.

This isn't the first downtick in U.S. life expectancy -- there have been three others since 1980, the most recent in 2005. It's too soon to say whether the current slight decline is the beginning of a plateau or whether American life span will resume its upward trend.

The report finds that from 2007 to 2008:

Life expectancy for white men is down 0.2 years to 75.3, but up 0.2 years to 70.2 for black men.

Life expectancy for white women is down 0.2 years to 80.3, but steady at 76.8 for black women.

The gap in life expectancy between black and white Americans is down to 4.6 years.

The report also looked at causes of death in 2007 and 2008:

Death rates from stroke are down 3.8%, dropping stroke to the fourth leading cause of death.